1. Field of the Invention
The present invention relates to an apparatus and method for finding exact locations for probing integrated circuits, and conversely tracing the circuit element and location of a suspected spot on a DUT (device under test) probed by a probing tool.
2. Description of the Related Art
Probing systems have been used in the art for testing and debugging integrated circuit (IC) designs and layouts. Various laser-based systems for probing IC's are known in the prior art. In these prior art systems, the DUT is driven by an electrical test signal, while a laser beam is used to illuminate the DUT. The laser beam then reflects from the DUT, and the reflection is perturbed according to the DUT's response to the electrical test signals. The reflected beam is then converted to an electrical signal having a waveform corresponding to the reflected beam. This waveform is displayed for the user's analysis.
Some of the test and debug technique used in the prior art include LIVA (Light Induced Voltage Alteration), TIVA (Thermally Induced Voltage Alteration), CIVA (Charge Induced Voltage Alteration), XIVA (Externally Induced Voltage Alteration), OBIC (Optical Beam Induced Current), OBHIC (Optical Beam Heat Induced Current), and OBIRCH (Optical Beam Induced Resistance Change). These techniques probe the DUT to detect a change in the characteristics of certain devices or structures therein to thereby detect a failure or an area that is prone to fail or adversely affect the DUT's performance. According to these techniques, the DUT is driven by an electrical signal, while a laser beam is used to illuminate the DUT to thereby cause either heating, carrier generation, or both. As a result, the electrical output from the DUT is perturbed, and this perturbation is detected and analyzed. That is, under these techniques the laser beam is used only as a perturbing agent, but the detection is done by analyzing the electrical output from the DUT.
Other probing techniques are based on the observation that active devices, such as transistors, emit light upon switching of state. In these systems, the DUT is also energized by test vectors, but no light is used to illuminate the DUT. Rather, an optical system is used to collect the faint light that is emitted upon the switching of the transistor being probed. This techniques is generally referred to in the industry as time-resolved emission (TRE) or time-resolved photon emission (TRPE). A system for performing TRPE probing is commercially available under the trade name EmiScope™ from Credence Systems Corporation of Milpitas, Calif.
As is known in the art, in order to probe the DUT and obtain meaningful analysis, the location of the probed devices must be specified as accurately as possible. This is especially true for TRPE, as the location from which emission emanates is very small, so that accurate placement of the collection optics is imperative for proper emission collection and proper identification of the device that emits the photons.
FIG. 1A is a schematic of prior art method of identifying probing locations on a DUT. In order to test a DUT, first test patterns (also referred to as test vectors) are generated by an automatic test pattern generator, ATPG 105. The test patterns are then fed to the automated testing equipment (also referred to as automated testing and evaluation) ATE 110. The ATE feeds the test vectors to the DUT, 145, and tests the electrical response of the DUT to the test patterns. The ATE then generates a log of fail results, 115, identifying scan chains and test patterns having failures. The log, 115, is then input to an analysis tool, 120, which performs fault simulation using the log so as to determine potential cells or pins that may be faulty. As is shown by the broken-line double arrow, some commercially available ATPG's contain mechanism to also perform the analysis, so while two separate tools are shown, the generation of test vectors and the analysis of the fail log can be done by the same tool. The analysis tool performs fault simulation to determine potential causes for the fault and then outputs a callout log, 125, which lists all of the cell names and related pins that are likely to result in detection of failures.
The cells and pins identified in the callout list may not be the actual cause of the failures. Therefore, it is a practice in the art to use a debug tester, such as a TRPE tool to investigate the actual cause for the fault. For that purpose, the callout log is used to determine which locations to probe with the TRPE tool. That is, using the callout list a debug engineer needs to perform various manual procedures, 150, to determine where to place the prober. The manual procedure involve reviewing various design databases, 130, reviewing the layout vs. schematic, LVS 135, and reviewing layout designs, 140, so as to provide the results, 160, normally in terms of the coordinates of the corner of the cell to be probed.
As can be understood, the methods used in the prior art to arrive at probing locations are tedious and time consuming. Additionally, in most instances only the coordinates of the corner of the cell to be investigated is obtained, and the prober needs to be scanned over the cell to find the actual proper location for probing. Therefore, there is a need in the art for an automated system that provides more detailed information regarding the locations to be probed.
Conversely, in cases where a fault location is detected by the prober, it is currently difficult to determine the corresponding circuit element that is probed. That is, the prober provides only the coordinates of the probing location with respect to the DUT. However, the prober provides no information with respect to layout or circuit design of the DUT, so that the circuit element that emits the light is not readily identifiable. FIG. 1B depicts a general process of the prior art for determining the circuit element that may be responsible for the emission spot. As shown, the location, 100, in terms of X,Y CAD coordinates on the DUT is obtained from the prober. The engineers go manually through various databases, such as design databases 130, layout vs. schematics database 135, and layout tools 140, to determine the candidate elements, 160, that may be the cause. Therefore, there is a need in the industry to provide a system and method to trace back a coordinate location of a probing location to a circuit element in the DUT layout and design.